Extreme Ultraviolet (EUV) lithography is a lithography technique that targets very small feature sizes (e.g., 32 nm node and below). EUV light may be produced using a small, hot plasma that will efficiently radiate at a desired wavelength (e.g., in a range of approximately 11 nm to 15 nm). Due to such short wavelengths, the use of refractive optics in the system is ineffective. Thus, reflective mirrors are to be implemented for the optics. To promote the reflectivity at EUV wavelengths, multilayer (ML) coatings, featuring alternating layers of molybdenum and silicon (Mo/Si) or molybdenum and beryllium (Mo/Be), are used.
ML mirrors typically used in current EUV optical systems have a constant thickness Mo/Si (e.g., similar to a Bragg's reflector). The ML includes 40-80 bi-layers of the Mo/Si pair to enhance EUV reflections. Further, the bi-layer has a constant period of 6.9 nm with approximately 2.8 nm of Mo and approximately 4.1 nm of Si. However, a problem exists for this type of ML mirror in that it has approximately 12° of the maximum acceptance angle. At this maximum angle, an approximate 16% reflectivity loss occurs with approximately 20° of phase shifts.
Such a small acceptance angle often imposes a significant limitation in designing high numerical aperture (NA) EUV optical systems. This limitation requires optical designers to use multiple mirrors with high asphericity. For a typical six-mirror system (with a NA higher than 0.25), it is possible to have pupil apodizations with large phase shifts (adding out-of-plane distortion (OPD) in the system) due to the acceptance angle limitations of the current ML mirrors. Thus, for high NA optics, more number of mirrors have to be used. Also, graded ML mirrors are sometimes employed for azimuthally symmetric optical element in the system.